a friend of mine did this going about mph because his brakes failed and he was going to rear end people waiting at a red light he had a pontiac firebird a very heavy car he said when he threw it into park nothing happened immediately but after a couple seconds the tranny started making a loud clicking then the rear wheels locked this managed to slow the car down a bit before the crash but he still totaled it he actually had time to choose which of the cars he was going to plow into a hard decision to make but better than a tbone collision im sure